U.S. patent number 4,180,790 [Application Number 05/864,597] was granted by the patent office on 1979-12-25 for dynamic array aperture and focus control for ultrasonic imaging systems.
This patent grant is currently assigned to General Electric Company. Invention is credited to Charles E. Thomas.
United States Patent |
4,180,790 |
Thomas |
December 25, 1979 |
Dynamic array aperture and focus control for ultrasonic imaging
systems
Abstract
A B-scan ultrasonic imager such as a single-sector scanner has a
dynamic aperture and focus control to attain improved lateral
resolution especially at ranges less than the maximum array
aperture. As the range from which echoes are being received
propagates out, the array aperture during each echo reception
period is increased by steps by switching in more elements of the
total transducer array. At least one adjustment of receiving
channel time delays is made to dynamically focus the echoes at
different focal points.
Inventors: |
Thomas; Charles E. (Scotia,
NY) |
Assignee: |
General Electric Company
(Schenectady, NY)
|
Family
ID: |
25343627 |
Appl.
No.: |
05/864,597 |
Filed: |
December 27, 1977 |
Current U.S.
Class: |
367/7; 367/105;
73/626 |
Current CPC
Class: |
G01N
29/0672 (20130101); G01N 29/262 (20130101); G01S
7/52046 (20130101); G01S 15/8918 (20130101); G10K
11/345 (20130101); G10K 11/346 (20130101); G01S
15/8927 (20130101) |
Current International
Class: |
G01N
29/06 (20060101); G01S 15/00 (20060101); G01S
15/89 (20060101); G01N 29/26 (20060101); G01S
7/52 (20060101); G10K 11/00 (20060101); G10K
11/34 (20060101); G01S 009/66 () |
Field of
Search: |
;340/1R,9
;73/626,629 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Thurston et al., Acoustical Holography, vol. 5, 1974, Plenum Press,
N. Y., pp. 249-259..
|
Primary Examiner: Farley; Richard A.
Attorney, Agent or Firm: Campbell; Donald R. Cohen; Joseph
T. Snyder; Marvin
Claims
The invention claimed is:
1. An ultrasonic imaging system with improved lateral resolution
comprising:
a linear array of transducer elements which are selectively
operative during alternate transmission and echo reception periods
to sequentially generate pulses of ultrasound for scanning an
object region and to generate received echo electrical signals,
a plurality of transmitting channels for producing excitation
pulses for selected array elements, and a plurality of receiving
channels connected to the array elements for delaying the echo
signals for variable predetermined time delays to focus the
received echoes,
aperture control means for increasing the number of active array
elements and receiving channels as a function of range during each
echo reception period such that a central group of receiving
channels are initially active and thereafter pairs of receiving
channels, one on either side, are additionally rendered active
whereby the array aperture is increased by steps at least during
the interval the range does not exceed the maximum aperture,
means for adjusting the receiving channel time delays at least once
during each echo reception period to dynamically focus the received
echoes at a plurality of focal points at different ranges, and
means for summing the delayed echo signals from all active
receiving channels to thereby generate a focused echo signal.
2. The imaging system of claim 1 wherein said pairs of receiving
channels are rendered active as a function of range during the
interval the range is less than the maximum array aperture until
the range is several times the maximum array aperture.
3. A single-sector scanner ultrasonic imaging system with improved
lateral resolution comprising:
a linear array of transducer elements which are selectively
operative during alternate transmission and echo reception periods
to sequentially generate angulated acoustic beams steered at many
angles for scanning an object region and to generate received echo
electrical signals,
a plurality of transmitting channels for producing excitation
pulses in time sequence for selected array elements, and a
plurality of receiving channels connected to the array elements for
amplifying and delaying the echo signals for predetermined steering
and focusing time delays to focus the received echoes,
aperture control means for symmetrically switching in and
increasing the number of active array elements and receiving
channels as a function of range during each echo reception period,
at least during the interval the range is less than the maximum
array aperture, by initially having a central group of active
receiving channels and thereafter switching in successive pairs of
receiving channels, one on either side, to increase the array
aperture by steps until the total number of receiving channels are
active,
means for adjusting the receiving channel focusing time delays at
least once during each echo reception period to dynamically focus
the echoes at a plurality of focal points at different ranges,
and
means for summing the delayed echo signals from all active
receiving channels to generate a focused echo signal and for
displaying the focused echo signals as a visual image of the
insonified object region.
4. The imaging system of claim 3 wherein said aperture control
means is operative to switch in the total number of receiving
channels at ranges greater than about three times the maximum array
aperture.
5. A method of ultrasonic imaging comprising the steps of:
exciting a linear array of transducer elements to sequentially
generate pulses of ultrasound for scanning an object region, said
transducer elements during subsequent echo reception periods being
operable as receive elements and generating received echo
electrical signals,
processing said echo signals in parallel channels wherein the echo
signals are delayed by variable amounts in the different channels
to focus the received echoes,
changing the number of active signal processing channels and
effectively active receive elements as a function of range during
each echo reception period by initially having a central group of
active receiving channels and thereafter switching in successive
pairs of receiving channels, one on either side, such that the
array aperture is increased by steps,
adjusting the signal processing channel time delays at least once
during each echo reception period to dynamically focus the echoes
at a plurality of focal points at different ranges, and
summing the delayed echo signals from all active signal processing
channels to generate a focused echo signal.
6. The method of claim 5 wherein adjustment of the channel time
delays to focus the echoes is coincident with a step change in
array aperture.
Description
BACKGROUND OF THE INVENTION
This invention relates to a method and apparatus for ultrasonic
imaging, and more particularly to a dynamic electronically
controlled aperture in combination with dynamic focusing for
improved lateral resolution in sector scanners and other B-scan
ultrasonic imaging systems.
One of the most important parameters determining ultrasonic image
quality is that of lateral resolution, which refers to a minimum
separation at which two targets can be distinguished in the
direction of the longitudinal axis of the linear transducer array.
Under far field conditions lateral resolution improves as the
transducer array aperture increases, whereas under near field
conditions lateral resolution improves as the transducer array
aperture decreases. It is known that focusing can theoretically
establish far field conditions in the near field, so that large
aperture would be an advantage at all ranges. However, it is
difficult to maintain focus at ranges less than the array aperture
even with electronic dynamic focus. In B-scan tomographic cardiac
imaging, ranges less than the array aperture are often of interest.
Thus, there are important applications where the optimum aperture
varies with depth across the image field.
The single-sector scanner is a real time imaging system having a
linear transducer array as depicted in FIG. 1, and is described as
a cardiac scanner by Thurstone and von Ramm in "A New Ultrasonic
Imaging Technique Employing Two-Dimensional Electronic Beam
Steering," Acoustical Holography, Vol. 5, 1974, Plenum Press, New
York, pp. 249-259. To make a sector scan, the elemental transducers
are excited in linear time sequence to generate angulated acoustic
beams at many angles relative to the normal to the array at its
midpoint. Echoes returning from targets in the direction of the
transmitted acoustic beam arrive at the transducer elements at
different times necessitating relative delaying of the received
echo electrical signals by different amounts so that all the
signals from a given point target are summed simultaneously by all
elements of the array. In addition to beam steering delays, dynamic
electronic focusing to improve image quality is achieved by
additional channel-to-channel delay differences to compensate for
propagation path time delay differences from a focal point to the
various individual array element positions. The beam steering and
electronic focusing delays are additive, and the focus can be
changed dynamically to increment the range from which echoes are
being received during a reception period. In the prior art sector
scanners the entire array of receive elements are active during an
echo reception period and the received signals from all receive
elements are delayed and summed to generate a focused echo signal
or video signal. That is, the array aperture during each echo
reception period is unchanged and is the maximum possible aperture.
At ranges less than the maximum aperture, dynamic focusing delays
must be changed so rapidly that it becomes nearly impossible to
keep up.
SUMMARY OF THE INVENTION
To achieve improved lateral resolution in an ultrasonic imaging
system with a linear array of transducer elements, especially at
ranges less than the full array aperture, the array aperture is
increased as the range from which echoes are being received
increases by effectively switching in more array elements by steps
during every echo reception period. The dynamic aperture control
for best image quality is combined with a dynamic focus control for
adjusting time delays in the echo signal processing channels to
focus the echoes at a plurality of focal points at different
ranges. At longer ranges, several times the full array aperture,
all of the array elements and echo processing channels are switched
in or are active for the best realizable lateral resolution.
The exemplary embodiment is an improved signal sector scanner for
real time medical imaging in which there is a switch, or equivalent
mechanism for blanking receiving channels individually, between
every array element and associated receiving channel. Initially,
only the central group of elements and receiving channels are
active during the echo reception period, and as the range increases
the elements and receiving channels are switched on symmetrically
by two's, one at either side, until all the elements are active and
the maximum aperture is attained. Changes in aperture size are made
during the interval the range is less than the maximum aperture
until the range is several times the maximum aperture, and at least
one adjustment of focusing time delays in the receiving channels is
made, the steering time delays remaining the same. The delayed echo
signals from the varying number of active receiving channels are
summed to generate the focused echo signal or raw video signal
which is displayed as a visual image of the insonified object
region on a cathode ray tube monitor. The dynamic aperture and
focus control is also applicable to the multi-sector scanner and
digital rectilinear scanner, both of which are described in
identified copending patent applications.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sketch illustrating operation of a single-sector
steered beam ultrasonic imager;
FIG. 2 schematically depicts time delay steering and focusing of
received echo signals from a receive array of transducer
elements;
FIG. 3 shows a simplified receiving channel diagram for a system
with dynamic aperture and focus control; and
FIG. 4 is a functional block diagram of a single-sector scanner
imaging system incorporating the dynamic aperture and focus
control.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The system for dynamic control of array aperture during every echo
reception period of a B-scan ultrasonic imaging system can
theoretically be used with any imaging system that has an array of
transducer elements for echo reception, whether that system employs
no focusing, mechanical focusing, fixed electronic focusing, or
dynamic electronic focusing. There are certain advantages, however,
in the combination of a dynamic array aperture control with a
dynamic focusing control in medical instrumentation for cardiology
and laminography, where good images with improved lateral
resolution in the near field region are needed, and in ultrasonic
imagers for similar industrial applications. The dynamic array
aperture and focus control can be used with both rectilinear and
sector scan systems, and with systems that have or do not have
array apodization. The preferred embodiment is a single-sector
scanner for imaging the beating heart in real time.
The single sector steered beam ultrasonic scanner in FIG. 1 has a
linear transducer array 10 comprising equally spaced elementary
transducers 11 which are energized by excitation pulses 12 in a
linear time sequence to form an ultrasound beam 13 and direct the
beam in a preselected azimuth direction to transmit a pulse of
ultrasound. The linear transducer array is also referred to as the
phased array. In order to steer the beam electronically to an angle
.theta. degrees from the normal to the array longitudinal axis, a
time delay increment
where c is the sonic velocity, is added successively to each i th
signal as one moves down the array from one end (i=1) to the other
(i=N) to exactly compensate for the propagation path time delay
differences that exist under plane wave (Fraunhofer) conditions. By
progressively changing the time delay between the successive
excitation pulses, the angle .theta. at one side of the normal is
changed by increments. To form and steer the beam at the other side
of the normal, the timing of excitation pulses 12 is reversed so
that the bottom transducer in FIG. 1 is energized first and the top
transducer is energized last. The total sector scan angle is
approximately 60.degree. to 90.degree.. Echoes returning from
targets 14 in the direction of the transmitted beam arrive at the
transducer elements at different times necessitating relative
delaying of the received echo electrical signals by different
amounts so that all the signals from a given point target are
summed simultaneously by all elements of the array. Within the
focal region in the near field the acoustic beam actually appears
as if it were in the far field as in FIG. 1, and this is further
explained in [Electronic Scanning of Focused Arrays] by V. G.
Welsby, Journal of Sound Vibration (1968), Vol. 8, No. 3, pps.
390-394. The magnitudes of the time delays of the individual echo
electrical signals are the same as during the transmission
operation to compensate for acoustic path propagation delay
differences. These are referred to as the beam steering time
delays, or simply steering delays.
In B-scan imaging focusing is not essential but improves image
quality by increasing resolution and reducing some kinds of
artifact problems. Electronic focusing, like beam steering, is
accomplished by the use of channel-to-channel electronic signal
delay differences to compensate for propagation path time delay
differences from the focal point to the various individual array
element positions. The electronic focusing delay increment for each
sub-array is given by
where
a=the sub-array half aperture distance,
f=the focal distance,
c=the sonic velocity,
x.sub.k =the distance from the sub-array center to the k th
element, and
T.sub.k =the time delay associated with the k th element.
It has been shown that the beam steering and focusing time delays
are additive, i.e., if one applies the time delay set required to
steer the beam to an angle .theta. and then adds the time delay set
required to focus at a range R, the focal point will be located at
range R measured along an axis .theta. degrees from the normal to
the longitudinal axis of the sub-array. The receiving focus, unlike
the transmitting focus, can be dynamically changed to track the
range from which echoes are being received during the echo
reception period by a one step or multi-step approximation. The
additive nature of steering and focusing time delay sets is an
approximation that is good except at very short ranges.
In FIG. 2, the steering and focusing time delays are indicated
separately in the receiving channels associated with array
transducer elements 11, and are depicted as rectangular blocks
which vary in length from channel to channel. To effect coherent
summation of the contributions from all active receive elements,
the delayed echo signals from the multiple receiving channels are
fed to a summing amplifier 15 at the output of which is a focused
echo signal or raw video data. After being processed through a scan
converter to convert the sector scan format to raster format, the
focused echo signal controls the electron beam intensity of a
cathode ray tube or television monitor as the image is built up
scan line by scan line. This is further explained in detail with
regard to FIG. 4. The steering time delays remain the same during
each echo reception period for a transmitted and steered pulse of
ultrasound, and are changed between echo reception periods to
correspond to the angulation of the next transmitted acoustic beam.
The focusing time delays, on the other hand, are adjusted
dynamically during every echo reception period to focus the
received echoes at a plurality of focal points as the range
propagates out. A typical operation to scan the object for one
image frame is that the transducer elements of array 10 are
energized in sequence with the delays initially set to generate a
transmitted acoustic beam at the farthest possible clockwise
position, the received echo signals being delayed in the same order
to steer the channel signals with additional relative delays for
focusing the echoes. In succeeding transmit-receive cycles, the
relative channel-to-channel time delays are progressively changed
to rotate the generated acoustic beam by small angular increments
in the counterclockwise direction toward the farthest possible
counterclockwise position. During each transmit-receive cycle, the
additional relative channel delays that control the focal range are
dynamically changed so that the focal range tracks the
echo-generation region. This monotonic counterclockwise increase of
the acoustic beam direction is not essential; the beam steering
delays can be controlled to select beam directions in any desired
order. A frame rate of about 30 frames per second is needed to
prevent blurring of the image of the portion of the heart being
pictured on the television screen.
In addition to changing the focusing delays at least once during
every echo reception period, improved lateral resolution in the
image is attained by dynamically controlling the array aperture as
a function of range such that the aperture is increased by steps at
ranges less than the maximum array aperture and normally extended
up to ranges several times the maximum array aperture. At ranges
further out approaching or in the far field, lateral resolution
improves as the transducer array aperture increases and there is
benefit in using the maximum possible array aperture. Under far
field conditions, the reflected echo acoustic wave fronts arriving
at the transducer array are plane or approximately plane. Focusing
can theoretically establish far field conditions in the near field
so that a large aperture would be an advantage at all ranges, but
in practice it is difficult to maintain focus at ranges less than
the array aperture even with dynamic electronic focusing. Aperture
control to improve lateral resolution at short ranges is
implemented by structuring the receiver so that the individual
channels can be blanked electronically with the possible exception
of a central group of receiving channels. The number of active
receive elements and receiving channels is then increased
symmetrically by steps until the maximum array aperture is reached
with all the receive elements and receiving channels active.
The dynamic aperture control is shown schematically in FIG. 2 as
pairs of receiver channel switches 16a-16e which are closed in
sequence during the echo reception period, one pair at a time, by
an aperture control circuit 17. Receiving channel switches 16a-16e
operate at high speed and are actually electronic switches, but the
function of blanking the channels can be performed in an equivalent
manner by reducing the amplifier gain or otherwise. At the shortest
ranges, well within the maximum array aperture, only the four
central array transducers are connected to their respective
receiving channels and the delayed echo signals in only these four
channels are summed to generate the focused echo signal. As the
range increases, switches 16a, one on either side of the central
group of four, are closed, and then pairs of switches 16b-16e are
closed in sequence, thereby incrementally increasing the size of
the receiver sub-array and the number of active receiving channels
whose delayed echo signals are summed by summing amplifier 15. As a
rule of thumb in the near field, beam width is approximately equal
to the size of the aperture, and lateral resolution varies with the
size of the aperture and therefore is best when the aperture is
small.
For cardiac scanning, linear transducer arrays can typically have a
length of 40 millimeters or 4 centimeters, and the width of the
elementary transducer is between 0.5 and 1.0 millimeter. There can
be more receive elements than transmit elements, and they need not
be equally spaced. For these dimensions, the maximum array aperture
is 4 centimeters and it is seen that details of a heart being
examined at ranges of less than 4 centimeters are often of
interest. The maximum range can be 25 centimeters, and it is
desirable to have the aperture control up to about one-half the
maximum range or 12 centimeters. This is three times the full
aperture. The frequency with which changes in the focusing time
delays to vary the focal length must be made, consistent with the
objective of good picture quality, also decreases as the range is
increased. The dynamic aperture and focus control is illustrated in
more general form in FIG. 3 with the addition of details of the
aperture control circuit. This system has an analog switch 18 and a
variable time delay device 19-1 or 19-2 such as selectable-delay
delay line in each array element receiving channel. The steering
and focusing delays for a single sector steered beam scanner can be
combined in the same device, as for instance a charge coupled
device (CCD) delay line comprised of two or more subdelay lines
whose delay times are separately controlled. The transmit pulse
trigger starts a timer 20 whose function is to blank out the array
transducer elements for a brief interval at ranges very close to
the array to allow reverberations to decay. At the end of a preset
interval an echo propagation distance computer 21 begins to
calculate the range and required time delays based on the elapsed
time for two-way propagation of sound and the velocity of sound in
tissue, about 154,000 centimeters per second. Computer outputs
generated at predetermined ranges control both an analog switch set
controller 22 and a delay set controller 23. As the range from
which echoes are being received propagates out, the array aperture
is increased in steps by switching in more elements of the total
transducer array. Shortly after pulsing the central group of
transducers 11-1, the associated analog switches 18 are closed to
connect the elements to the receiving channels and the delays of
devices 19-1 are adjusted for a focus at f.sub.1 and the echoes are
coming from range r.sub.1. When propagation distance computer 21
indicates that echoes are being received from range r.sub.2,
elements 11-2 and delay devices 19-2 are connected in the receiver
circuit with the delays adjusted to provide a focus at f.sub.2. If
array apodization is used, it is desirable to adjust element
weights when the aperture is adjusted. For the case where there are
more elements in the total transducer array, the aperture is
increased symmetrically in steps of two. A central group of
transducers is ordinarily either two or four elements.
FIG. 4 is a system block diagram of the single sector scanner
ultrasonic imager with provision for dynamic aperture and focus
control at short ranges in the near field for improved lateral
resolution. The linear transducer array is illustrated with only
four transducer elements 25a-25d, but in practice the array has a
larger number of elements, some of which may be receive only
elements. The four transmitting and receiving channels 26a-26d are
each comprised by level and timing control circuitry 27 under the
control of a master digital controller 28 for determining the level
and timing of a pulse generated by transmit pulser 29 and applied
to one of the transducer elements. The receiving channel for
processing the received echo electrical signal is comprised by an
analog switch 30, a preamplifier 31 having a limiter to protect the
sensitive preamplifier inputs from the high transmitting voltage,
and a compression amplifier 32 to reduce the larger dynamic
acoustic range to the smaller range a cathode ray tube display
device can handle. The amplified echo signal is next fed to a
digitally selected analog delay device 33 having associated delay
selection circuitry 34 which, under the control of digital
controller 28, presets the delay to steer and focus the echo signal
in that channel. The other three receiving channels are identical
except for the values of the time delays employed. Digital
controller 28 can take various forms and can be a hard wired logic
circuit but is preferably a properly programmed microcomputer or
minicomputer incorporating the functions of the aperture control
(timer, echo distance computer, and controllers) in FIG. 3. In
operation, transducer excitation pulses are generated in the four
transmitting channels in time sequence to steer the generated
ultrasound beam and control the scan angle. During the echo
reception period, the central two analog switches 30 are closed,
after a short interval to reject reverberations, and the received
echo signals generated by elements 25b and 25c are delayed by
different preset amounts to steer and focus the echoes at a first
focal point. At a predetermined range, the two outer switches are
closed to increase the array aperture by switching in elements 25a
and 25d, and the focusing time delays are adjusted to focus the
echoes at a second focal point. The delayed echo signals from all
active receiving channels, either two or four, are fed to a summing
amplifier 35 to generate the focused echo signal which is presented
to a scan converter 36 before being displayed. The scan converter
is described in copending application Ser. No. 853,347, filed on
Nov. 21, 1977 by J. J. Tiemann, entitled "Scan Converter For
Ultrasonic Sector Scanner" and assigned to the same assignee as
this invention. Other types of scan converters such as an analog
storage tube can also be used. The scan converter controls sweep
drivers 37 and the generated X and Y deflection signals for cathode
ray tube 38 on the screen of which is displayed, in real time, the
single sector image. During every succeeding echo reception period
following generation of an acoustic beam at a different scan angle,
the array aperture is dynamically increased in combination with
adjusting the focusing time delays and focal point.
The dynamic aperture and focus control can be employed in other
ultrasonic imaging systems with a linear array of transducer
elements. It is applicable to imaging systems that do not use scan
converters, but instead cause the movement of the beam of the
cathode ray tube to directly follow the movement of the acoustic
echo region in the propagating medium in real time. It is directly
applicable without further explanation to the multi-sector scanner
disclosed in allowed copending application Ser. No. 825,528, filed
on Aug. 18, 1977 by H. A. F. Rocha jointly with the present
inventor. This system has a longer linear transducer array for
producing a set of sector scans with the origin points of the
sequential sector scans displaced longitudinally along the array. A
digital rectilinear ultrasonic imaging system is described in U.S.
Pat. No. 4,127,034 granted on Nov. 28, 1978 to F. L. Lederman and
J. J. Tiemann. This new architecture has a linear transducer array
wherein the elements are pulsed one at a time while alternately
storing the received echo signals in a long, segmented digital
delay line memory having a number of delay lines equal to the
number of elements in a receive sub-array. After N pulses, the N
transducers in the array have been selected, and their outputs are
simultaneously present at the N taps of the segmented delay line.
Upon shifting the receive element echo data from one delay line to
the next, the echo data read out of all delay lines are also
processed through parallel channels where the data is subjected to
focusing delays and then fed to a summing amplifier for coherent
summation. In this case, an array aperture control is effected by
blanking out processing channels before coherent summation so that
the number of channels fed to the summing amplifier and the number
of receive elements contributing to the focused echo signal
increased by steps as a function of range. Both of the foregoing
applications are assigned to the same assignee as this
invention.
To summarize, the method of ultrasonic imaging for improved lateral
resolution and picture quality comprises processing the echo
signals from separate receive elements in parallel channels for
delaying the echo signals by variable amounts in the different
channels to focus the received echoes; changing the number of
contributing active signal processing channels and therefore the
number of effectively active receive elements as a function of
range during the echo reception period to increase the array
aperture by steps; adjusting the processing channel time delays at
least once during the echo reception period to dynamically focus
the echoes; and continuously summing the delayed echo signals from
all active signal processing channels.
The components of an ultrasonic scanner with the dynamic aperture
and focus control can be standard integrated circuits or
conventional circuitry as is presently known in the art. Further
information on sector scanners for real time imaging is given in
the previously referenced publications and patent applications.
While the invention has been particularly known and described with
reference to preferred embodiments thereof, it will be understood
by those skilled in the art that various changes in form and
details may be made therein without departing from the spirit and
scope of the invention.
* * * * *